1. Field of the Invention
This invention relates to new and useful improvements in supported silver catalysts, their method of preparation and use, particularly, in the manufacture of aldehydes such as formaldehyde.
2. The Prior Art
The use of silver as a catalyst for many different types of reactions is well known. Silver used in catalytic reactions can be in the form of solid silver crystals or the silver can be coated on to an inert substrate to produce a supported silver catalyst. Generally, such supported silver catalysts are on porous refractory spheres, rings, pellets and the like. Supported silver catalysts find application in a number of chemical reactions such as hydrogenation, cracking, dehydrogenation and the production of ketones and aldehydes from primary and secondary alcohols. Silver is an expensive material and a variety of techniques have been developed for depositing relatively small amounts of silver on refractory material and the use of such silvered material in place of solid silver.
U.S. Pat. No. 3,956,184 of May 11, 1976 to Kruglikov, et al., and U.S. Pat. No. 4,126,582 of Nov. 21, 1978 to Diem, et al., describe preparation of supported silver catalysts for use in the manufacture of formaldehyde. The supported silver catalysts of those patents suffer from a number of shortcomings, particularly their method of manufacture which relies on the use of strong chemical reducing agents. I find that the use of the reducing agent techniques of these two patents is not practical because of the large quantities of solutions required in the silver deposition techniques which also coat the equipment with silver. There is a residual silver powder suspension in the solution to be filtered out and the residual strongly alkaline solution containing toxic reducing agents has to be disposed. Much the same problem exists with the extensive washing needed and disposal of the contaminated wash water. The silver on the equipment has to be removed prior to the next batch or the silver will preferentially deposit on the existing silver surface. If promotors are to be added to the supported catalyst in order to optimize the reaction equilibrium, the reduction methods of these two patents are not suitable for homogeneous distribution of the promoter metal unless a compound of the promoter is used which reduces under the same conditions as the silver. Also, particularly in the case of 184 patent, the silver would penetrate deep into the carrier and thus much of the silver would not be available for the catalytic reaction.
U.S. Pat. No. 2,462,413 mentions the use of a supported silver catalyst for the preparation of formaldehyde. The catalyst is placed, as a bed, in insulated silica tubes wherein from about 0.5 to about 10% of silver is deposited on the carrier such as pumice or alumina-silica refractories. This patent does not describe: preparation of the support prior to silver deposition; how the silver is deposited on the support; and little information is given as to other properties of the silver coated support.
U.S. Pat. No. 3,702,259 prepares supported silver catalysts by use of oranic amine solubilizing and reducing agents for depositing silver on porous refractory supports. The catalyst preparation methods of this invention as well as the catalyst itself, suffer from much the same shortcomings as the before mentioned methods, e.g., use of toxic reducing agents and loss of silver by penetration into the porous carrier. Although the principal utility of the catalyst of this 259 patent is for the manufacture of ethylene oxide, the patent does mention utility for a variety of chemical reactions, including the preparation of aldehydes from primary alcohols.
U.S. Pat. No. 2,805,229 shows the preparation of supported silver on porous refractory tubes. The silver is deposited on the tube surfaces from an ammoniated silver solution which is subsequently dried on the tubular carrier and then heated at about 250.degree. C. to 350.degree. C. to decompose the silver compounds to metallic silver. It appears that both the inside and outside surfaces of the tubes are coated with silver and the silver penetrates into the porous carrier. The tubular catalysts of this patent are described as useful for the oxidation of ethylene to ethylene oxide.
U.S. Pat. No. 3,725,302 of Apr. 3, 1973 to Brown, et al., shows the preparation of supported silver catalyst for the production of ethylene oxide. This patent mentions the use of spherical supports wherein the silver is deposited throughout the pores of the particle by immersing the support in an aqueous solution of a silver salt, drying the support and finally heating the support at a temperature sufficient to decompose the silver salt. Optionally, an alkaline earth promoter is added to the silver salt solution.
U.S. Pat. No. 3,981,825 of Sept. 21, 1976 to W. J. Raegan shows metallic reforming catalysts prepared by mixing a solid porous support with a solution of various soluble metal compounds in dimethylsulfoxide, subjecting the resulting slurry to refluxing, separating the solid carrier from the slurry, drying the carrier and finally calcining the metal compound impregnated carrier.
U.S. Pat. No. 2,424,083 describes the preparation of supported silver catalysts together with promoter metals wherein the silver solution is contacted with the support and reducing agents convert the metal salts to a metallic coating on the support or carrier.
U.S. Pat. No. 2,765,283 of Oct. 2, 1956 to Sacken shows the preparation of supported silver catalysts wherein the support is first washed with a chlorine solution to deposit small quantities of chlorine compound on the support. Following a wash of the chlorine containing support, the carrier particles may be dried. The support is then contacted with silver oxide, such as silver oxide paste and then dried, and finally the silver oxide is reduced to metallic silver.
U.S. Pat. No. 1,067,665 of July 15, 1913 describes preparation of silver catalysts by precipitating a silver salt from an ammoniated solution, applying the salt to various supports and apparently finally calcining the salt on the support. Prior to contact with the silver salt, the support is also calcined to rid it of organic matter.